Electrically-operated machines perform a wide range of functions, from the whimsical to the life-saving. All such machines share the common characteristic that they will not operate correctly without electrical power. Accordingly, as the purpose a machine serves becomes more important, so too are the measures taken to ensure that the machine's power source will not be interrupted.
General- and special-purpose computing devices commonly require direct current (DC) power at several voltages. These voltages are produced by a system component called a power supply, which typically converts electrical energy from the alternating current (AC), high voltage form in which it is often delivered, to the lower DC voltages used by the device. Power supplies are rated in terms of the maximum power they can supply at their output voltages. The cost of a power supply is roughly proportional to its rated power: a 100 watt (W) supply may cost approximately half as much as a 200 W supply.
In order to protect a computing device against power failures, a common technique is to install two or more identical power supplies with their individual DC outputs paralleled, each capable of providing the full amount of power required by the device, and each connected to a separate incoming power circuit. This approach is shown in FIG. 2. There, a first power supply 140 may be connected to an ordinary wall socket 110 providing power at 120 VAC or 240 VAC at 50 or 60 Hz, while a second power supply 160 may be connected to an uninterruptible power supply (UPS, not shown), through connection 120. The UPS may provide a similar voltage generated from energy stored in batteries. If either the wall socket or UPS fails, or if either the first or second power supply fails, the computing device will continue to receive the power it needs.
Unfortunately, providing fully-redundant power supplies in this manner is expensive: two supplies cost twice as much as one. Furthermore, although the incoming power circuits may be indistinguishable in terms of their voltage and frequency, one may be more expensive to use than another. For example, a power supply that draws its input continuously from a UPS may require a more capable (and more expensive) UPS, or may cause the UPS's batteries to wear out more quickly.